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   难度：Medium
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   <h1 class="question_title">
    445. Add Two Numbers II
   </h1>
   <p>
    You are given two
    <b>
     non-empty
    </b>
    linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.
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   <p>
    You may assume the two numbers do not contain any leading zero, except the number 0 itself.
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   <p>
    <b>
     Follow up:
    </b>
    <br>
    What if you cannot modify the input lists? In other words, reversing the lists is not allowed.
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   <p>
    <b>
     Example:
    </b>
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   <pre>
<b>Input:</b> (7 -&gt; 2 -&gt; 4 -&gt; 3) + (5 -&gt; 6 -&gt; 4)
<b>Output:</b> 7 -&gt; 8 -&gt; 0 -&gt; 7
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   <h1 class="question_title">
    445. 两数相加 II
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   <p>
    给定两个
    <strong>
     非空
    </strong>
    链表来代表两个非负整数。数字最高位位于链表开始位置。它们的每个节点只存储单个数字。将这两数相加会返回一个新的链表。
   </p>
   <p>
    &nbsp;
   </p>
   <p>
    你可以假设除了数字 0 之外，这两个数字都不会以零开头。
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   <p>
    <strong>
     进阶:
    </strong>
   </p>
   <p>
    如果输入链表不能修改该如何处理？换句话说，你不能对列表中的节点进行翻转。
   </p>
   <p>
    <strong>
     示例:
    </strong>
   </p>
   <pre>
<strong>输入:</strong> (7 -&gt; 2 -&gt; 4 -&gt; 3) + (5 -&gt; 6 -&gt; 4)
<strong>输出:</strong> 7 -&gt; 8 -&gt; 0 -&gt; 7
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